1. Field of the Invention
The present invention relates to a method of reading out signals from a solid-state image pickup apparatus in which photosensitive cells for photoelectric transduction are arranged in a so-called honeycomb pattern.
2. Description of the Background Art
Generally, a solid-state image pickup apparatus is apt to generate moire and other false signals. Japanese Patent Laid-Open Publication No. 136391/1998, for example, discloses a solid-state image pickup apparatus having pixels densely arranged so as to increase the received quantity of incident light, thereby promoting efficient receipt of light. The apparatus taught in this document provides a new structure that optimizes spatial sampling of an image and is referred to as a honeycomb pixel arrangement.
In the honeycomb pixel arrangement, assuming that the distance between nearby pixels on the same row or the same column is a pitch, then pixels around a given pixel each are shifted from the given pixel by half a pitch in the direction of row and/or the direction of column. In a CCD (Charge-Coupled Device) type of solid-state image sensor using the honeycomb pixel arrangement, vertical transfer registers constitute vertical transfer paths extending zigzag in such a manner as to skirt round the pixels. Color filter segments are assigned to actually existing ones of the pixels. The pixels, or photosensitive cells, photoelectrically transduce light incident thereto through the color filter segments to thereby generate signal charges having color attributes. The signal charges are then sequentially routed through the vertical transfer registers and horizontal transfer registers, which constitute a horizontal transfer path perpendicular to the vertical transfer path, to an output amplifier. The output amplifier performs Q/V conversion for outputting voltage signals in the form of analog signals.
The analog signals thus generated are subjected to signal processing. First, a correlation between pixel data is determined with consideration given to the colors of actual pixels. More specifically, pixel data of a color in question closely correlated to each other are used to estimate, by calculation, pixel data at a virtual pixel, defined as a pixel at which the actual pixels are not positioned, and pixel data at actual pixels of a color different from the color in question. Subsequently, one of such pixel data appearing in a direction closer in correlation than the remaining pixel data is used for interpolating virtual pixels. Such interpolation successfully reduces false signals. Further, the pixel data are successfully broadened in frequency band, enhancing resolution.
It has also been proposed to use the honeycomb pixel arrangement for further enhancing the resolution of an image and broadening the dynamic range of image signals generated. In accordance with a specific conventional scheme directed toward this object, the photosensitive area of the individual photosensitive cell is divided into a main region and a subregion different in size from each other, so that signal charges are read out from the two regions independently of each other. This scheme broadens the dynamic range on the basis of a difference in sensitivity between the main and subregions. In a usual signal reading mode, the signal charges of the main and subregions are mixed together and read out in the conventional manner.
Processing for broadening the dynamic range is determined by the sensitivity difference between the photosensitive regions and the saturation in a pair of photosensitive regions.
In order to broaden the dynamic range, it is a common practice to design the solid-state image pickup apparatus so as to simply read out a signal charge from the main region of the individual photosensitive cell and then read out a signal charge from the subregion of the same photosensitive cell in the interlace fashion. Image data derived from the signals sequentially read out from the main and subregions are in turn added to broaden the reproducible range of the quantity of incident light.
The conventional solid-state image pickup apparatus has the following problems left unsolved. By preliminary image shooting or pickup that precedes actual image shooting or pickup, the image pickup apparatus performs photometry and then executes AE (Automatic Exposure) and AF (Automatic Focus) control with optics, thereby estimating the luminance distribution of a scene to be picked up. A problem is that, despite that a scene to be captured sometimes does not require the processing for broadening the dynamic range, depending upon the luminance distribution thus estimated, the apparatus is adapted to read out signal charges from the main and subregions in the interlace fashion without exception. Today, to meet an increasing demand for, e.g. a digital camera having a larger number of pixels and therefore higher image quality, efforts are being made toward shorter signal reading time and higher signal processing speed. This, however, requires a high voltage for processing and thereby aggravates power consumption.
On the other hand, a digital camera is powered by a battery so as to implement a small size, light weight, portable configuration. Thus, the reduction of power consumption and a power supply available with a digital camera are contradictory to each other. Because priority is, in practice, given to the effective use of a battery, a device for saving power is the target to tackle.